CFRCs are known in the art. They generally comprise a matrix, such as a polymer resin, for example an epoxy or other polymer, and a reinforcement of carbon fibers. The carbon fibers might also contain other fibers, such as aluminum or glass. Structures of various aerospace applications are often made in whole or in part of CFRC.
The initiation and propagation of damage ultimately results in failure of aerospace structural components. Typical structural repairs often result in damaging practices, where material is ground away and holes are drilled to secure patches, which can act as new sites for damage. By healing known damage or by providing healing material to areas that are suspected to incur damage, improved safety can be realized. Damage-tolerant, self-healing structural systems provide a route towards this objective. Effective self-repair, however, requires that these materials heal quickly following low- and mid-velocity impacts, while retaining structural integrity. Although there are materials known to possess this characteristic, such is not the case for structural engineering systems.
Self-healing materials display the unique ability to mitigate incipient damage and have built-in capability to substantially recover structural load transferring ability after damage. Structures that make use of self-healing engineering materials produce a healing response from a change in the material's chain mobility as a function of the damage mechanism/condition involved. This type of material will possess better mechanical properties, healing capability at elevated temperatures, faster healing rates (less than 100 microseconds), and healing without the need of foreign inserts or fillers (via structural chemistry). These materials might have application as structural aerospace applications.
In recent years, researchers have studied different “self-healing mechanisms” in materials as a collection of irreversible thermodynamic paths where the path sequences ultimately lead to crack closure or resealing. Crack repair in polymers using thermal and solvent processes, where a healing process triggered with heating or with a solvent has been studied. A second approach involves the autonomic healing concept, where healing is accomplished by dispersing a microencapsulated healing agent and a catalytic chemical trigger within an epoxy resin to repair or bond crack faces and mitigate further crack propagation. A related approach, the microvascular concept, utilizes brittle hollow glass fibers in contrast to microcapsules filled with epoxy hardener and uncured resin in alternate layers, with fluorescent dye. An approaching crack ruptures a hollow glass fiber, releasing a healing agent into the crack plane through capillary action. A third approach utilizes a polymer that can reversibly re-establish its broken bonds at the molecular level by either thermal activation (e.g., based on Diels-Alder rebonding), or ultraviolet light. A fourth approach, structurally dynamic polymers, are materials that produce macroscopic responses from a change in the materials' molecular architecture without heat or pressure. A fifth approach, self-healing fiber-reinforced composites, involves integrating self-healing resins into fiber reinforced composites. Various chemistries have been used based on some of the aforementioned approaches described above. Although significant recovery (>90 percent) of virgin neat resin material properties have been reported, this range has not been the case for fiber-reinforced composites made from them.
The aforementioned self-healing approaches that address the repair or mitigation of crack growth and various damage conditions in materials, have the following disadvantages: (1) Slow rates of healing; (2) Use of foreign inserts in the polymer matrix that may have detrimental effects on composite performance; (3) Samples have to be held in intimate contact or under load and/or fused together under high temperature for long periods of time; and (4) The material may not be considered a structural load-bearing, material.
For example, ionomers containing ionic groups at low concentrations (<15 mol percent) along the polymer backbone. In the presence of oppositely-charged ions, these ionic groups form aggregates that can be activated by external stimuli such as temperature or ultraviolet irradiation. One such ionomer, poly(ethylene-co-methacrylic acid) [EMAA], also known under the trade name Surlyn®, undergoes puncture reversal (self-healing) following high-velocity ballistic penetration. The heat generated from the damage event triggers self-healing in this material. Although EMAA polymers possess excellent puncture healing properties, their low tensile modulus (308 MPa) limits their use as an engineering polymer in structural aerospace applications.
Also, a self-healing composite laminate system that possesses aerospace quality consolidation with fiber volume fraction (FVF) of up to 57 percent and void volume fraction of less than two percent does not currently exist. Most self-healing composite laminates that have been reported possess 20-30 percent fiber volume, which is well below aerospace industrial standards for fiber-reinforced composites (FRC).
A need exists for an inherently self-healing composite laminate matrix that does not rely on foreign inclusions for self-repair and which has mechanical properties with potential for aerospace applications. A need further exists for an appropriate process for making such a matrix, for an appropriate process for making CFRCs from such a matrix, and for repairing such CFRCs.